Suchada Rajca

Impact of Electron-Electron Spin Interaction on Electron Spin Relaxation of Nitroxide Diradicals and Tetraradical in Glassy Solvents Between 10 and 300 K

To determine the impact of electron-electron spin-spin interactions on electron spin relaxation rates, 1/T1 and 1/Tm were measured for nitroxide monoradical, diradical, and tetraradical derivatives of 1,3-alternate calix[4]arenes, for two pegylated high-spin nitroxide diradicals, and for an azine-linked nitroxide diradical. The synthesis and characterization by SQUID (superconducting quantum interference device) magnetometry of one of the high-spin diradicals, in which nitroxides are conformationally constrained to be coplanar with the m-phenylene unit, is reported. The interspin distances ranged from about 5-9 A, and the magnitude of the exchange interaction ranged from >150 to >0.1 K. 1/T1 and 1/Tm were measured by long-pulse saturation recovery, three-pulse inversion recovery, and two-pulse echo decay at X-band (9.5 GHz) and Q-band (35 GHz). For a diradical with interspin distance about 9 A, relaxation rates were only slightly faster than for a monoradical with analogous structure. For interspin distances of about 5-6 A, relaxation rates in glassy solvents up to 300 K increased in the order monoradical < diradical < tetraradical. Modulation of electron-electron interaction enhanced relaxation via the direct, Raman, and local mode processes. The largest differences in 1/T1 were observed below 10 K, where the direct process dominates. For the three diradicals with comparable magnitude of dipolar interaction, 1/Tm and 1/T1 were faster for the molecules with more flexible structures. Relaxation rates were faster in the less rigid low-polarity sucrose octaacetate glass than in the more rigid 4:1 toluene/chloroform or in hydrogen-bonded glycerol glasses, which highlights the impact of motion on relaxation.

Radical Cation of Helical, Cross-Conjugated β-Oligothiophene

The radical cation of carbon-sulfur [7]helicene is configurationally stable in solution at room temperature. In contrast to the radical cations of alpha-oligothiophenes, which form diamagnetic pi-dimers at low temperature, the radical cation of this helical, cross-conjugated beta-oligothiophene shows a low propensity toward dimerization.

Synthesis and Reduction Kinetics of Sterically Shielded Pyrrolidine Nitroxides

A series of sterically shielded pyrrolidine nitroxides were synthesized, and their reduction by ascorbate (vitamin C) indicate that nitroxide 3, a tetraethyl derivative of 3-carboxy-PROXYL, is reduced at the slowest rate among known nitroxides, i.e., at a 60-fold slower rate than that for 3-carboxy-PROXYL.

Triplet Ground State Derivative of Aza-m-xylylene Diradical with Large Singlet−Triplet Energy Gap

Organic molecules with a strong preference for triplet ground states, in which the triplet state is below the lowest singlet state by ≥10 kcal/mol, are typically short-lived and mostly detected as reactive intermediates. We now report a triplet ground state derivative of aza-m-xylylene diradical with a large singlet-triplet energy gap (ΔE(ST)) of ∼10 kcal/mol, which is comparable to ΔE(ST) for the well-known reactive intermediate m-xylylene diradical. The aminyl diradical persists in solution at room temperature on the time scale of minutes.

Noncovalent Interactions in the Asymmetric Synthesis of Rigid, Conjugated Helical Structures

Tetrakis({beta}-trithiophene) 1 folds into a helical conformation (RRR) that facilitates double ring annelation, with high diastereoselectivity and modest enantioselectivity, to provide bis[7]helicene 2 (MRM). This rigid, helically locked structure has enhanced chiroptical properties similar to the corresponding [15]helicene.

A Spirocyclohexyl Nitroxide Amino Acid Spin Label for Pulsed EPR Spectroscopy Distance Measurements

Site-directed spin labeling and EPR spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide alpha-amino acid and its N-(9-fluorenylmethoxycarbonyl) derivative, have been synthesized, which exhibit slow enough spin-echo dephasing to permit accurate distance measurements by pulsed EPR spectroscopy at temperatures up to 125 K in 1:1 water/glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR spectroscopy to the study of structure and conformation of peptides and proteins.

Stable diarylnitroxide diradical with triplet ground state

Nitroxide diradical , the first isolated diarylnitroxide diradical, is stable in the solid state at room temperature and it possesses triplet ground state with strong ferromagnetic coupling.

A Greek cross dodecaphenylene: sparteine-mediated asymmetric synthesis of chiral D2-symmetric π-conjugated tetra-o-phenylenes

Abstract The asymmetric synthesis of a chiral, nonracemic π-conjugated system with D2 point group of symmetry, dodecaphenylene 4 is described. In the key step, (−)-sparteine- and Cu(II)-mediated oxidation of 2,2′-dilithio-1,1′-biaryls in ether gives the corresponding dimers, tetra-o-phenylenes, in 80% isolated yields and 50–60% ees. X-Ray crystallography confirms the structure of rac-4 and its molecular shape of a Greek cross. The torsional angles between the benzene rings in the tetra-o-phenylene core of rac-4 are in the 56.5–71.0° range. However, CD and UV spectra of 4 in CH2Cl2 are consistent with significant conjugation between the four terphenyl moieties.

High-Spin S = 2 Ground State Aminyl Tetraradicals

Aminyl tetraradicals with planar tetraazanonacene backbones have quintet (S = 2) ground states and do not show any detectable thermal population of the low-spin excited states up to the highest temperature investigated (100 K) in the 2-methyltetrahydrofuran (2-MeTHF) matrix. This indicates that the nearest electronic excited state (triplet) is at least ~0.3 kcal mol(-1) higher in energy, that is, the triplet-quintet energy gap, ΔE(TQ) > 0.3 kcal mol(-1), which is consistent with the broken-symmetry-DFT-computed ΔE(TQ) of about 5 kcal mol(-1). In concentrated (ca. 1-10 mM) solutions of tetraradical 4 in 2-MeTHF at 133 K, a fraction of tetraradicals form a dimer (association constant, K(assoc) ≈ 60 M(-1)), with a weak, antiferromagnetic exchange coupling, J/k ≈ -0.1 K ~ 0.2 cal mol(-1), between the S = 2 tetraradicals. This weak intradimer exchange coupling is expected for two tetraradicals at the distance of about 6 Å. The most sterically shielded tetraradical 5 in 2-MeTHF has a half-life of 1 h at room temperature; the product of its decay is the corresponding tetraamine, suggesting that the hydrogen atom abstraction from the solvent is primarily responsible for the decomposition of the tetraradical.

Isolation of the Triplet Ground State Aminyl Diradical

Nitrogen-centered (aminyl) radicals are typically short-lived, and have been detected as intermediates in a variety of chemical and biological processes. 2] To the best of our knowledge, only two known stable aminyl radicals have been reported (more than 30 years ago), 4] while a few others have recently been isolated as aminyl metal complexes and as heteroatom (e.g., O, S) stabilized aminyls. Aminyl diradicals, in which the radical centers are connected through m-phenylene to form a planar crossconjugated p system devoid of stabilizing heteroatoms, are predicted by computational studies to possess a strong preference for a high-spin, triplet (S = 1) ground state, that is, a singlet–triplet energy gap (DEST) that is an order of magnitude greater than the thermal energy (RT) at room temperature. 9] Such a nitrogen-centered diradical with large DEST values and persistence at room temperature may be relevant to the design of strongly paramagnetic relaxation reagents as well as the development of magnetic materials and devices. 13] To date, the two known examples of aminyl diradicals have been examined only at very low temperature. Platz and coworkers first reported an aminyl diradical in a 2-methyltetrahydrofuran (2-MeTHF) matrix at about 195 8C. Recently, we prepared an aminyl diradical with an S = 1 ground state that was persistent in solution at about 100 8C. Herein we report the isolation of an S = 1 ground state aminyl diradical with a large DEST value and persistence at room temperature. The synthesis of the aminyl diradicals 1 and 2 demonstrates the balance between maintaining an effective 2pp–2pp overlap that enhances electron spin–spin interactions (exchange coupling), and the adequate protection of the carbon and nitrogen atoms with significant spin densities within the diazapentacene backbone in order to achieve the stability required for isolation. This result is significant as the 2pp–2pp overlap is generally severely perturbed by the increased steric bulk that is required for achieving radical stability, and thus a combination of a large DEST value and persistence at room temperature is not commonly accomplished. In addition, we detected an unexpected self-associated dimer of the S = 1 aminyl diradical 2 in 2-MeTHF solution; this species appears to resemble a p dimer. Notably, characterization of the dimer is facilitated by its unique zero-field splitting tensor (D). p Dimers of neutral organic radicals in solution are relatively rare and the reports of such phenomena are limited to those of S = =2 radicals. Diradicals 1 and 2 were prepared from the corresponding diamines 5 and 6, which were obtained from diamine 3 (Scheme 1). The structure of diamine 6 was confirmed by X-ray crystallography, and shows overcrowding of the bulky tert-undecyl groups so that the C C bond axes that connect the two outer pendant benzene rings to the backbone bend outward (Figure 1). This bending creates more space for the pendant benzene rings in order to attain moderate torsion angles of 52.0(4), 61.0(4), and 68.2(3)8 with respect to the diazapentacene backbone. Nevertheless, both nitrogen atoms are well-shielded and the diazapentacene backbone remains planar in spite of the steric congestion. The structure of diradical 2, which differs from its diamine precursor by the absence of two hydrogen atoms, is expected to be similar to that of diamine 6. Such a sterically shielded, planar structure should have enhanced stability and a strong preference for the S = 1 ground state. In spite of hindered access to the NH moieties in the diazapentacene backbone, double deprotonation of diamines 5 and 6 in THF/hexane at about 30 8C, followed by oxidation of the resultant dianions with iodine between 90 and 115 8C provided the corresponding diradicals 1 and 2, respectively. Generation of 1 is difficult because of its low solubility, and characterization of 1 is possible only by EPR spectroscopy, which gave spectra that are typical of the aminyl diradical in the S = 1 state (Figure S3 in the Supporting Information). In contrast, the generation of 2 is very efficient, and is facilitated by the good solubility of the diradical and its diamine precursor. Diradical 2 was prepared on a 1 mg (1 mmol) scale in custom-designed sample tubes, thus enabling sequential measurements by EPR spectroscopy and superconducting quantum interference device (SQUID) magnetometry at low temperatures. The EPR spectra of concentrated purple-blue solutions of 2 in 2-MeTHF at 132 K are strikingly complex, [*] Dr. P. J. Boratyński, Prof. Dr. S. Rajca, Prof. Dr. A. Rajca Department of Chemistry, University of Nebraska Lincoln, NE 68588-0304 (USA) Fax: (+ 1)402-472-9402 E-mail: arajca1@unl.edu Homepage: http://www.chem.unl.edu/rajca/rajcahome.html